Exomoons: A new avenue for astrobiology

A team of astronomers may have discovered the first known moon beyond the Solar System, and it is not playing by the rules. Finding exoplanets – that is, planets orbiting stars other than our Sun – is a tough task. But a greater challenge is determining if these worlds have their own satellites – or ‘exomoons’.

Some of the most promising locations for extra-terrestrial life within our own Solar System can be found on the moons orbiting the giant planets Jupiter and Saturn. The potential existence of exomoons orbiting exoplanets would therefore expand the number of worlds available for life exponentially.

So far, however, the count of exomoons sits at zero – but that may be about to change with a recent discovery.

On 29 October, the Hubble Space Telescope steered its mighty gaze towards a star 4000 lightyears away in the hope of confirming the existence of an exomoon for the first time. First announced back in July 2017, the potential moon, known as Kepler-1625 b-i, was discovered within observations taken by NASA’s Kepler Space Telescope.

The Kepler telescope hunts for planets by searching for dips in the brightness of a star caused by a planet crossing in front; events known as transits. To search for an exomoon, researchers look for a minute dimming of the light from the star before and after the planet causes its own light dip. This technique is akin to spotting the dimming caused by a fly passing in front of a lightbulb several miles away, and then trying to spot a flea on the fly’s back.

Nonetheless, a promising signal was finally observed during three transits of the exoplanet, and now a recent study hopes to place constraints on the exomoon’s true nature. The study, conducted by René Heller, an astrophysicist from the Max Planck Institute for Solar System Research in Germany, examined data obtained by the Kepler mission and found that the host planet is a roughly Jupiter-sized brown dwarf (a failed star/pseudoplanet).

Most interestingly, however, is that estimates suggest the exomoon could be anything from a small gas world with the mass of Earth, up to a Neptune-sized gas giant. If true, this will be the first instance where a gas giant has been discovered orbiting another gas giant. “This exomoon, if real, is most likely gaseous,” Dr. Heller explained. “The Solar System’s moons are all rocky/icy bodies. So how would a gas giant moon have formed? I don’t know. I don’t know if anybody knows.”

The moons in our Solar System are thought to have formed through three known methods: something large impacted a planet and jettisoned material into orbit; the combination of material already orbiting a planet; or a passing object was captured by a planet’s gravity. If the exomoon exists, it is therefore unlike any of the moons in our Solar System. By determining the size and makeup of the potential moon, it is likely that there will be a race between space research groups from all around the world to find a theory capable of explaining the moon’s formation.

More observations are needed to confirm the existence of the potential moon, and this is where the Hubble Space Telescope plays a crucial role. When Kepler-1625 b-i made its most recent transit on 29 October, Hubble was watching. Based on this new data, scientists should now be able to get a better understanding of whether or not this mysterious moon really does exitst and what it looks like.

Studying the new Hubble data could take several months, however, and there have been several cases in the past where instrument glitches have provided false positives, so Kipping and his team remain cautious. Confirming the existence of an exomoon – however large it ends up being – will ultimately open up a whole new avenue for astrobiology research.

Jupiter’s Europa and Saturn’s Enceladus both have liquid water buried under an ice crust, so potentially inhabitable exomoons are not out of the question. It’s just a matter of time before we will be able to find them.

*A note on nomenclature. Exoplanet systems are such that the host star gets index a, and the planets b, c, etc. So, Kepler-1625 a indicates the star, and Kepler-1625 b the exoplanet. For a moon, an additional numeral suffix is added. In this case, Kepler-1625 b-i is the potential exomoon.*